Benchmark Sensex plunged over 232 points to end at 34,616.13 on Monday, extending its fall to the fifth straight session over political and macroeconomic concerns.
Investors were wary after BJP's B S Yeddyurappa resigned as the chief minister of Karnataka on Saturday minutes before he was to face a floor test to prove his contested majority in the hung assembly, paving the way for the JD(S)-Congress combine to form the government.
Sentiment got another jolt after Moody's Investors Service downgraded PNB's rating, citing the impact of recent fraud on its capital as well as weak internal controls, brokers said.
The 30-share Sensex had soared about 126 points to hit a high of 34,973.95 in early trade but gave up its gains following a widespread sell-off, which dragged it down to 34,593.82.
The gauge finally ended at a nearly one-month low of 34,616.13, down 232.17 points or 0.67 percent. This is its weakest closing since April 25 when it had finished at 34,501.60.
It had lost 708.41 points in the previous four sessions.
The broader NSE Nifty closed lower by 79.70 points, or 0.75 per cent, at 10,516.70, after hovering between 10,621.70 and 10,505.80.
A plunge in the rupee's value and sustained foreign fund outflows added to the gloom.